I have a mountain bike where the chain intermittently skips when put under load on the middle (and very rarely on the smallest) chain ring. It is not related to shifting.

I've taken several steps to correct it including:

calibrated front and back gear shifters

replaced the chain

replaced the rear gears

replaced the middle gear at the pedals (which was the most affected)

To my great frustration the problem is still there! Could this be because of the derailer? The chain seems tight enough. Help from any gurus out there would be greatly appreciated.

Update: I've tried fiddling with the back screw on the back-derrailer (not the L/H-screws) in the hope that it was a chain tension issue but it did not help. I've also checked the chain for stiff links but could not find any. I am not able to reproduce it when not riding the bike so I am not certain where and how the chain skips. I am at my wits end and planning to bring it to a different repair shop with a sensai mechanic so hopefully he will be able to sort it out.

chain length (how to tell). If there's an extra link in your chain the extra slack in the lowest gear might be enough to let the chain skip. Especially if you're not in the largest rear cog at the time. Technique matters here - if you mostly use the lowest "granny gear", then shift back to the middle chainring rather than onto smaller rear cogs this "too much chain" problem will not happen.

frame damage, specifically cracks around the head tube, down tube and chainstays. In the smallest chainring is when the forces on the frame are highest, so problems will show up in this gear first. Clean the bike, then carefully check the frame for damage. If the paint is chipped or cracked paint over it (with paint or nail varnish, anything that dries hard) then do a test ride under the conditions that cause the problem and check the new paint for cracks. Any new cracks mean there's too much flex, and might mean underlying frame damage. Or possibly just that you're too heavy/strong for the frame.

try adjusting the rear cable while riding the bike. Frame flex is probably the issue here, and doing the adjustment with your weight on the bike might make the difference

try re-routing the gear cables in crossover rather than parallel. The gear cables cross under the down tube - the rear cable starts on the left hand side of the head tube. This may mean new cables. This addresses shifting while you're out of the saddle swinging the bike from side to side (lateral bending rather than vertical).

new gear cables and outers. This should have been done earlier if they're not in perfect condition. Even if they look perfect, if they're more than half a year old I'd replace them just because it'd be really annoying if this fixes the problem after I'd spent ages trying other things. It's ~$20 of parts, so it's cheap compared to what you've already spent.

With a new chain a sticky link really shouldn't be an issue, but just a question: you do lube your chain, right? Especially if this happens off road, a dirty chain might cause issues.

'''Edit: middle chainring, not smallest'''

That does change things. The "too much chain" option is unlikely, as is frame damage. I would really have to put the bike in a workstand at look at exactly what happens when you're using different rear cogs and the middle chainring. Also, check for wear/slop in the rear derailleur (can you push it back and forth without shifting the gear lever). Unfortunately it's in the "I'm not sure what exactly I would be looking for, just anything that doesn't look right".

Going to a different mechanic, especially one know to be an expert, is about all I can really suggest. Sorry.

Sorry for any confusion, I think my use of imprecise terms might have led to edits that changed what I meant. The problem occurs most frequently on the middle chain-ring regardless of which back cogs are in use.
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Hans LøkenMay 30 '11 at 7:30

Thank you for all that information, lots of stuff I need to check when I get home!
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Hans LøkenMay 30 '11 at 7:42

The problem is likely due not to anything from the chain or the cogs, but rather cable tension.
If the chain is trying to shift onto bigger (lower gear) cogs, then the cable is too tight. The chain will keep trying to catch the next larger cog.
If it's too loose, it will try to shift to the next smaller cog. (presuming a standard derailleur setup, some are "backwards")
Try this.
Shift to the smallest cog. Take hold of the cable at the frame and see if you can pull it easily out further. If so, it's too loose. If it won't move at all, and feels tight, it's too tight.
Loosen the cable at the derailleur, and pull it so that it's just snug. That should be pretty close.
If the problem persists, you may also check the derailleur hanger. That's the little piece of aluminum that the derailleur is bolted to, that attaches it to the frame. These are meant to be replaceable, and are frequently bent.

As far as I can tell the chain is centred on both back and front cogs with no pull either towards lower or higher gears. It does not happen when at the largest cog in front (which has the greatest radius and largest number of "teeth") which to me indicates that it might have something to do with chain-tension and/or cogs. The back-derailer (once again I apologize if the term is imprecise) is pretty old, maybe the spring is getting old?
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Hans LøkenMay 29 '11 at 15:34

Did you replace the gear cable? Also, to what extent is the screw at the back of the top of the rear derailleur inserted? (the one next to the dropout, not the L/H screws.) Are the jockey wheels clean? Is the chain of the correct length (will not be slack when on smallest chainring and smallest sprocket)? With a new chain you TYPICALLY need to take out 1-2 sets of links to get it the right length.
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ʍǝɥʇɐɯMay 29 '11 at 18:53

The chain and back cogs were changed by my regular bike repair shop, I assume they did it correctly, it looks okay (to me). Thank you for the tip in regards to the rear derailler screw. Will check that out when I get home.
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Hans LøkenMay 30 '11 at 7:34

I have been having a similar problem of ghost shifting on my full suspension mountain bike. This first started after breaking my chain on a ride.

When I replaced the chain, I also decided to replace my rear derailleur cables and their sleeves. After noticing the symptoms I ended up replacing the rear cassette based on advice from a bike shop.

Following that with no luck, it became very frustrating and I started to think about the exact circumstances when it would ghost shift:

Only when on the bike, and not while on the tuning stand.

Only after putting a big load on the peddles or when going over a big bump.

Then it dawned on me that perhaps it was connected the movement of the rear suspension on the bike.
I looked at the derailleur cable sleeve that runs underneath the bike and noticed that when I was standing on the bike, the sleeve which normally has some slack, gets tight against the frame due to the travel of the rear suspension while weighted.

I replaced the cable sleeve with a new segment with an additional 1" of length. After this, the sleeve no longer pinches against the frame, and the ghost shifting problem has gone away!

I had this problem on one bike and it turned out to be a problem with the derailler -- basically I had the wrong one. Some deraillers don't have enough "gear range" such that when the chain length is adjusted to avoid slack on the smallest cogs, they become too tight on the larger cogs and don't allow enough "wrap" on the rear cogs.

This happened on a bike where the airline wrecked the original Suntour triple and so I went with a (moderately) long-cage Shimano. It skipped regardless of what I tried, so I got a Huret Duopar (the Suntour being unavailable) and it worked like a charm (though not particularly "crisp" in shifting).

After seeing my sensai bike mechanic he pointed out that I'd attached my new middle chain ring the wrong way around. :-) However, I was doubting that it was causing the problem I was seeing (and also the same problem had been present with the old chainring) and being constrained timewise I decided to replace the back derailler as well. After replacing the derailler with a new SLX the a similar problem was still present on the middle chain ring but no longer present on the smallest chain-ring. I then flipped the middle chainring the right way around and the problem was gone! Wohoo!

So in conclusion I do believe the old derailler was the main culprit but can't be sure since I also messed about with the middle chainring.

Does it always happen when you are on the smallest 2 cogs in back. They wear out fast if you spend alot of time on them. Its common. You can produce enough torque in the smallest and middle front chain rings but not the bigger. I believe your back cogs are worn. Try experimenting when riding with the big and small cogs in back. I bet your little cogs are very worn (very sloped sides).